I find I need to pull my foreskin back behind my glans, roll the condom down over my glans and foreskin, then holding the condom and foreskin up behind my glans roll it down to the base. Trick is NOT to get foreskin between glans and condom or get the condom under the foreskin behind the glans otherwise it can be uncomfortable and even painfull. Once inside my gf my cock gets harder and holds the condon on. I also like to make sure it is tight around the base, I hate finding the condom has slid halfway down my cock as I pull out leaving a cum filled flat baloon hanging down.
I guess everyone is different.